Presently, the most widespread type of electric vehicle (EV) motor is an Interior Permanent Magnet (IPM) motor. This motor type has good power density, and the IPM's structure allows the motor to be operated in a constant-power “flux weakening” regime above the base speed (typically about 4000 RPM) up to the motor's maximum speed (typically about 10,000 RPM). However, permanent magnets are expensive, mechanically fragile, are damaged by high temperatures, and have previously faced supply problems. It is therefore desirable to have a motor that does not fundamentally depend on strong permanent magnets.
An alternative to IPM motors is the Induction Motor (IM) which has no permanent magnets. Induction motors are presently used by some electric vehicle vendors. The motors can be relatively power dense and can be readily operated with a reduced field above the motor's base speed. However, since high currents flow on the rotor, accommodation of rotor heating can be a design challenge. The power factor of an IM is also low relative to a PM machine.
Switched Reluctance Motors (SRMs) have sometimes been proposed for use in automotive applications. However, they have high torque ripple, and they are not very amenable to running in a field weakening mode. However, the SRM has a very simple construction that is inexpensive to build relative to other motor designs. Since all current flows on the stator, the machine is also relatively easy to cool.
Wound field heteropolar inductor machines have some of the attributes of an SRM (i.e., simple rotor), but they are potentially easier to control with a voltage source inverter because of their relatively sinusoidal back electromotive force (implying lower torque ripple). However, these machines have not been widely considered for automotive applications because they are viewed to have a very poor power factor. See I. Boldea et al., “Automotive electric propulsion systems with reduced or no permanent magnets: an overview,” IEEE Transactions on Industrial Electronics, 61(10):5695-5711, October 2014 incorporated herein by this reference.